VDI Mirroring failed, tapdisk fails

Live-migrating a VM and moving a single VDI of a VM of size 800G fails in XenCenter with "VDI Mirroring failed"

In /var/log/daemon.log at the end of the runtime of tapdisk with PID 1486 the following is noted:

Feb 28 12:27:50 xen02 tapdisk[1486]: Adding secondary image: nbd:mirror_vbd/13/xvdc
Feb 28 12:27:50 xen02 tapdisk[1486]: nbd: Opening nbd export to mirror_vbd/13/xvdc (flags=408)
Feb 28 12:27:50 xen02 tapdisk[1486]: nbd: Found passed fd. Connecting...
Feb 28 12:27:50 xen02 tapdisk[1486]: nbd: Got size: 858993459200
Feb 28 12:27:50 xen02 tapdisk[1486]: nbd: Got flags: 0
Feb 28 12:27:50 xen02 tapdisk[1486]: nbd: Successfully connected to NBD server
Feb 28 12:27:50 xen02 tapdisk[1486]: nbd: Opening in secondary mode: Read requests will be forwarded
Feb 28 12:27:50 xen02 tapdisk[1486]: opened image mirror_vbd/13/xvdc (1 users, state: 0x00000001, type: 15, rw)
Feb 28 12:27:50 xen02 tapdisk[1486]: In mirror mode
Feb 28 12:27:50 xen02 tapdisk[1486]: Added secondary image
Feb 28 12:27:50 xen02 tapdisk[1486]: nbd: NBD server unpause(0x12aba10) - fdrecv_listening_fd -1, unix_listening_fd=23
Feb 28 12:27:50 xen02 tapdisk[1486]: sending 'resume response' message (uuid = 1)
Feb 28 12:27:50 xen02 tapdisk[1486]: tap-err:nbd: Server shutdown prematurely in read_some
Feb 28 12:27:50 xen02 tapdisk[1486]: tap-err:nbd: Error reading reply header: -1
Feb 28 12:27:50 xen02 tapdisk[1486]: nbd: NBD client full-disable
Feb 28 12:27:50 xen02 tapdisk[1486]: nbd: Setting closed
Feb 28 12:27:50 xen02 tapdisk[1486]: tap-err:vbd: Got non-zero res for NBD secondary - disabling mirroring: xenvbd-13-51744.

iostat -x 10 on xen02 (source):

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util
sda 0.00 26.60 585.70 114.90 130963.10 8613.40 199.22 12.29 17.50 1.42 99.68
sda1 0.00 1.50 0.00 0.50 0.00 16.00 32.00 0.04 88.80 47.20 2.36
sda2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
sda3 0.00 25.10 585.70 96.80 130963.10 8597.40 204.48 12.24 17.90 1.46 99.68

await on sda is floating arount 16..23.

The destination has practically no load (await <1.0)